1. Field of the Invention
This invention relates to power combiners used with RF and microwave frequency signals in general and more particularly to a tunable combiner in which amplitude unbalance between the input and output ports can be minimized or eliminated.
2. Description of the Prior Art
Power combiners couple electromagnetic energy from multiple input ports to an output port. They are used in a number of applications such as combining two or more signals at the same or different frequencies for transmission by a common antenna or combining outputs of multiple power amplifiers.
Power combiners have been fabricated using printed circuit boards with stripline circuit lines. Stripline refers to a circuit line that is sandwiched between two grounded planes. The ground planes control the impedance of the circuit line. The stripline design provides low insertion loss.
Referring to FIG. 1, a conventional stripline combiner 20 is shown. Combiner 20 has an upper printed circuit board 22 mounted over a lower printed circuit board 26. Upper printed circuit board 22 has a top surface 23 and bottom surface 24. Lower printed circuit board 26 has a top surface 27, bottom surface 28 and ends 29 and 30. Top surface 23 is covered with a conductive metallized layer or area 25. Similarly, bottom surface 28 is covered with a conductive metallized layer or area 31. Metallized layers 25 and 31 serve as ground planes.
A circuit line or stripline 32 is formed on top surface 27. Circuit line 32 has ends 33 and 34. A circuit line or stripline 36 is also formed on top surface 27. Circuit line 36 has ends 37 and 38. A common line 40 is connected with ends 34 and 38. Port 42 is connected to common line 40. Port 44 is connected to circuit line end 33. Port 46 is connected to circuit line end 37. Ports 44 and 46 would be input ports and port 42 an output port. Conductive vias 48 extend from top surface 27 to bottom surface 28.
Unfortunately, etching of the circuit lines during manufacturing is uneven due to manufacturing process variables and tolerances. The uneven etching leads to the circuit lines having different line widths. The uneven line width causes impedance differences in the circuit lines and the insertion loss between the input port and output port to be different. The uneven line width also causes amplitude unbalance between the input port and output port. Amplitude unbalance degrades the electrical performance of the combiner. The amount of amplitude unbalance is an important parameter in the performance of the combiner.
While power combiners have been used, they have suffered from having a large amplitude unbalance. What is needed is a power combiner that can be tuned to provide a low amplitude unbalance over a wide range of frequencies.